I stand between God and you (Deuteronomy 5:5).
We can also read the verse to mean that it is the "I" that stands between God and you. Indeed, many commentaries make the illuminating interpretation that the ego not only forms the barrier between God and people, but it also separates us from our fellow men and women.
Self-centeredness renders us unable to empathize with others - to share in their distress or participate in their success. When we are completely preoccupied with ourselves, we lack the time and capacity to be attentive to others, and barriers to communication inevitably develop.
The great works of mussar and chassidus stress that people must efface themselves before God, because to the degree that they are occupied with their own importance, to that degree they separate themselves from God. Even sin cannot separate a person from God the way vanity does. It is of the vain person that God says, "I cannot coexist in his presence" (Sotah 5a).
Self-effacement does not mean low self-esteem. How? If people realize that their abilities are gifts from God, they can then be both humble and aware of their skills and talents.
If we allow awareness of our potential to go to our heads, however, we begin to consider others inferior to ourselves. Our hollow feelings of superiority not only disrupt our sense of belonging with others, but also cause the vanity and arrogance which repel the Divine Presence.